Compact dual-field IR2-IR3 imaging system

ABSTRACT

The invention relates to a dual-field (NF and WF) imaging system comprising an optronic detector ( 1 ) and an optical combination of narrow-field focal length F NF  having, an optical axis a front lens, a narrow-field entrance pupil situated in the vicinity of the front lens, a real wide-field entrance pupil, that is to say situated upstream of the front lens, an intermediate focal plane (IFP). The optical combination has, on the optical axis, the following refractive groups: a convergent front group G 1  of focal length F, where F&lt;F NF /2, this group G 1  comprising the front lens, a divergent field-change group G 2  that can move along the optical axis, this group being situated upstream of the IFP in NF configuration and downstream of the IFP in WF configuration, a relay group G 3  imaging the IFP on the focal plane of the detector. The imaging system has a cooled IR2/IR3 detector and since the refractive groups have lenses, at least three different materials including CaF 2  are used for the lenses of the front group G 1.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is based on International Application No.PCT/EP2007/063180, filed on Dec. 3, 2007, which in turn corresponds toFrench Application No. 0610871, filed on Dec. 13, 2006, and priority ishereby claimed under 35 USC §119 based on these applications. Each ofthese applications are hereby incorporated by reference in theirentirety into the present application.

TECHNICAL FIELD

The field of the invention is that of compact NF (narrow-field) WF(wide-field) infrared systems.

BACKGROUND OF THE INVENTION

An NF (3°×2.25°) and WF (9°×6.75°) camera open at F/2.7 with anarrow-field focal length F_(NF) equal to approximately 180 mm,wide-field focal length F_(WF) equal to approximately 60 mm, fitted witha multiple quantum well IR3 (7.5-10 μm) matrix detector is known.

The photometric behavior of this camera is optimized for thermalinfrared. Specifically:

-   -   the aperture diaphragm is materialized by the cold screen 3 of        the detector 1 (represented in FIG. 1) protected by its window        2,    -   there is no vignetting because the only element limiting the        beams is the aperture diaphragm.

This camera is very compact. Specifically:

-   -   the path L of the beam along the optical axis between the        entrance refractive surface and the focal plane (the detector 1)        is less than F_(NF)/1.2,    -   the section of the duct occupied by the optical beams is less        than F_(NF)/2.5 between a plane situated at F_(NF) mm upstream        of the entrance refractive surface and the focal plane.

This compactness is obtained by virtue of:

-   -   a narrow-field entrance pupil situated in the vicinity of the        front lens: this feature requires the presence of an IFP        (Intermediate Focal Plane) in the optical combination,    -   a real wide-field entrance pupil, that is to say situated        upstream of the front lens.

The optical combination of this camera comprises the followingrefractive groups described with reference to FIG. 1:

-   -   a. a convergent front group G1 of focal length F, where        F<F_(NF)/2, therefore extremely open,    -   b. a divergent field-change group G2 that can move along the        optical axis; this group is situated upstream of the IFP in NF        configuration and downstream of the IFP in WF configuration,    -   c. a relay group G3 imaging the IFP on the focal plane of the        detector.

The group G1 is achromatized in the IR3 band by virtue of one of theconventional sequences Ge(+)/ZnSe(−) or Ge(+)/ZnS(−) or Ge(+)/DOE(+),with Ge for germanium, ZnSe for zinc selenide, ZnS for zinc sulfide, DOEfor diffractive optical element, + for convergent and − for divergent.

The object of the invention is to be able to use such a camera and moregenerally an imaging system also in the IR2 band (3.5-5 μm) with asingle detector. The 2 bands are however not used simultaneously: theuseful band is selected by adjusting the polarization of the detector.

The existing camera, and more precisely the front group G1, hasconsiderable chromatic aberration in IR2 which compromises its use inthis spectral band.

SUMMARY OF THE INVENTION

The object of the invention is to obtain a compact dual-field IR2-IR3imaging system that does not have the abovementioned disadvantages.

The invention is based on the use for the lenses, notably those of thefront group, of a triplet of materials suited to the two spectral bands.The triplets used contain CaF₂, commonly called fluorine; they aretriplets ZnSe(+)/Ge(−)/CaF₂(−) or ZnS(+)/Ge(−)/CaF₂(−).

The subject of the invention is a dual-field (NF and WF) imaging systemhaving an optronic detector and an optical combination of narrow-fieldfocal length F_(NF), on an optical axis. The optical combination has afront lens, a narrow-field entrance pupil situated in the vicinity ofthe front lens, a real wide-field entrance pupil, that is to saysituated upstream of the front lens, and an intermediate focal plane(IFP).

The optical combination has, on the optical axis, the followingrefractive groups:

-   -   a convergent refractive front group G1 of focal length F, where        F<F_(NF)/2, this group G1 comprising the front lens,    -   a divergent field-change group G2 that can move along the        optical axis, this group being situated upstream of the IFP in        NF configuration and downstream of the IFP in WF configuration,    -   a relay group G3 imaging the IFP on the focal plane of the        detector.

The detector is a cooled IR2/IR3 detector, and in that at least threedifferent materials including CaF₂ are used for the lenses of therefractive front group.

The invention makes it possible to reduce the power of the lenses andtherefore their number while retaining a good optical quality, that isto say a good MTF. This then gives a compact dual-field IR2-IR3 imagingsystem.

Preferably, the lenses of the refractive front group G1 are based ontriplets ZnSe(+)/Ge(−)/CaF₂(−) or ZnS(+)/Ge(−)/CaF₂(−).

According to one feature of the invention, the relay G3 comprises atleast one diffractive lens (L3A).

According to another feature of the invention, the detector is a matrixor linear multiple quantum well detector.

Advantageously, since the imaging system has a cold screen, the latteris used as an aperture diaphragm and all vignetting is prevented becauseonly this diaphragm limits the optical beams.

Still other objects and advantages of the present invention will becomereadily apparent to those skilled in the art from the following detaileddescription, wherein the preferred embodiments of the invention areshown and described, simply by way of illustration of the best modecontemplated of carrying out the invention. As will be realized, theinvention is capable of other and different embodiments, and its severaldetails are capable of modifications in various obvious aspects, allwithout departing from the invention. Accordingly, the drawings anddescription thereof are to be regarded as illustrative in nature, andnot as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not bylimitation, in the figures of the accompanying drawings, whereinelements having the same reference numeral designations represent likeelements throughout and wherein:

FIG. 1 already described represents schematically the opticalcombination of a compact NF and WF camera according to the prior art,

FIG. 2 represents schematically one embodiment of an imaging systemaccording to the invention in NF and WF mode,

FIG. 3 represents schematically a tank gun sight comprising an imagingsystem according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The functions of the various elements of the optical combination of theimaging system according to the invention will now be described. Theseelements are generally identical to those of the camera described withreference to FIG. 1.

The refractive front group G1 images the scene on an intermediate focalplane IFP1.

This intermediate image, which is virtual in narrow-field mode and realin wide-field mode, is taken by the divergent group G2 (preferablycomprising a single lens L2), which gives an image on a secondintermediate image plane IFP2, which is real in narrow-field mode andvirtual in wide-field mode. The divergent lens L2 is placed before theintermediate image planes IFP1 and IFP2 in narrow-field mode, after theintermediate image planes IFP1 and IFP2 in wide-field mode. The twoplanes IFP1 and IFP2 are positioned in the same location for the twonarrow-field or wide-field configurations. The divergent lens images thefirst plane IFP1 onto the second IFP2 with a magnification of√(F_(NF)/F_(WF)) in narrow-field mode, and of 1/√(F_(NF)/F_(WF)) inwide-field mode. This function makes it possible to have a narrow-fieldfocal length F_(NF)/F_(WF) times larger than the wide-field focallength.

The second focal plane IFP2 is then taken by the relay group G3 ofmagnification of between approximately 1.3 and 1.5. Its object plane isreal.

The relay group G3 preferably comprises 2 lenses L3A and L3B as shown inFIG. 2.

Since the interband chromatism is not corrected, the IR2 and IR3intermediate focal planes of the front objective are not superposed. Thegroup G2, that can move on the axis in order to allow the change offield, also makes it possible to refocus the image onto the detectorwhen the user changes spectral band.

The axial movement of the group G2 also makes it possible to providerefocusing of the image when the temperature inside the imaging systemchanges or in order to observe at closer quarters.

In order to refine the thermal refocusing, at closer quarters or whenthere is a change of spectral band, one of the lenses of the group G3(preferably the lens L3A) may also be made movable on the axis.

L3B is preferably a microscanning lens which makes it possible totranslate the image of the scene on the detector by ½ pixel. Thisfunction makes it possible to increase the resolution of the imagingsystem.

This configuration makes it possible to obtain an NF-WF imaging system;the narrow-field entrance pupil is situated in the vicinity of the frontlens, and the wide-field entrance pupil is real, that is to say situatedupstream of the front lens, which makes it easier to install the imagingsystem in a sighting periscope system because this eliminates the riskof vignetting of the WF beams.

In this architecture, the chromatic aberrations of the front group G1are the largest.

Recalling the chromatism equations, consider an optical module, of powerφ, the axial chromatism of which it is desired to correct in each of thetwo spectral bands. This module comprises three lenses each made in adifferent material. Let A, B and C be the three lenses and let:

$\quad\left\{ \begin{matrix}{{\phi\; A},{\phi\; B},{\phi\; C\mspace{14mu}{be}\mspace{14mu}{the}\mspace{14mu}{respective}\mspace{14mu}{powers}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu} 3\mspace{14mu}{lenses}},} \\{{v\; 1A\mspace{14mu}{and}\mspace{14mu} v\; 2A\mspace{14mu}{be}\mspace{14mu}{the}\mspace{14mu}{IR}\; 2\mspace{14mu}{and}\mspace{14mu}{IR}\; 3\mspace{14mu}{constringencies}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{material}\mspace{14mu} A},} \\{{v\; 1B\mspace{14mu}{and}\mspace{14mu} v\; 2B\mspace{14mu}{be}\mspace{14mu}{the}\mspace{14mu}{IR}\; 2\mspace{14mu}{and}\mspace{14mu}{IR}\; 3\mspace{14mu}{constringencies}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{material}\mspace{14mu} B},} \\{v\; 1C\mspace{14mu}{and}\mspace{14mu} v\; 2C\mspace{14mu}{be}\mspace{14mu}{the}\mspace{14mu}{IR}\; 2\mspace{14mu}{and}\mspace{14mu}{IR}\; 3\mspace{14mu}{constringencies}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{material}\mspace{14mu}{C.}}\end{matrix} \right.$

The equations for the paraxial correction of the chromatism of a tripletare as follows:

$\quad\left\{ \begin{matrix}{\phi = {{\phi\; A} + {\phi\; B} + {\phi\; C}}} & \; \\{{{\phi\;{A/v}\; 1\; A} + {\phi\;{B/v}\; 1\; B} + {\phi\;{C/v}\; 1C}} = 0} & \left( {{correction}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{IR}\; 2\mspace{14mu}{chromatism}} \right) \\{{{\phi\;{A/v}\; 2\; A} + {\phi\;{B/v}\; 2B} + {\phi\;{C/v}\; 2C}} = 0} & {\left( {{correction}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{IR}\; 3\mspace{14mu}{chromatism}} \right).}\end{matrix} \right.$

These equations correct the chromatism in the IR2 band, and in the IR3band. There may however remain some interband chromatism: the IR2 andIR3 intermediate focal planes of the front objective are not superposed.

The triplet of materials ZnSe(+)/Ge(−)/ZnS(−) is known to ensure anachromatization in the IR2 and IR3 bands. But the use of this tripletleads to powers of ZnSe and ZnS lenses that are much greater than thepower of the front group. The user is therefore limited to front groupsthat are not very powerful or not very open or bulky. If, for example,the power of a triplet of lenses respectively made of ZnSe, Ge and ZnSis 1, the powers of these lenses are respectively approximately 2.7,−0.6 and −1.1. The aperture of the lens made of ZnSe is therefore 2.7times greater than the aperture of the front group which is already veryopen.

With the compactness constraints of the invention, the user then has toincrease the number of lenses (at least 7) in the front group in orderto limit the aperture of each of them to a reasonable value. This largenumber of lenses significantly increases the cost of the combination. Inaddition, although the modulation transfer function (MTF) obtained issufficient in theory, it is considerably degraded as soon as themanufacturing and lens mounting tolerances are taken into account.

According to the invention, the triplets ZnSe(+)/Ge(−)/CaF₂(−) orZnS(+)/Ge(−)/CaF₂(−) are used.

CaF₂ is rarely used in IR3 because it is absorbent for λ>10 μm, whereasmost of the IR3 detectors are sensitive up to 12 μm. The QWIP detectorused for this imaging system is not sensitive for λ>10 μm and thereforeallows the use of CaF₂.

Since CaF₂ has a very weak constringency relative to the other infraredmaterials, it makes the correction of chromatism very easy. If, forexample, the power of a triplet of lenses respectively made of ZnSe, Geand CaF₂ is 1, the powers of these lenses is respectively approximately1.62, −0.52 and −0.1. If, for example, the power of a triplet of lensesrespectively made of ZnS, Ge and CaF₂ is 1, the powers of these lensesare respectively approximately 1.64, −0.4 and −0.24. The whole value ofthis triplet relative to the conventional solution ZnSe(+)/Ge(−)/ZnS(−)can therefore be understood.

The greatest chromatism is corrected in the following manner whichrepresents one embodiment of the invention represented in FIG. 2.

The front group G1 comprises five lenses L1A and L1E.

L1A is preferably made of Ge, divergent, spherical. A lens made of Ge isplaced at the front in order to protect the imaging system from outsideattack, whether they be of electromagnetic or environmental nature.

The lens made of ZnSe is divided into L1B and L1D; L1B is aspherical inorder to correct the spherical aberration.

L1C is made of CaF₂.

The assembly L1A to L1D is corrected on the axial chromatism by virtueof the use of the ZnSe/Ge/CaF₂ triplet.

L1E is a germanium meniscus. It makes it possible to correct the fieldcurvature, and to help in the pupil conjugation in the narrow-fieldchannel.

The transmission of the CaF₂ lens is 96% in IR2 and 80% in IR3 takingaccount of the absorption of CaF₂ in this band and supposing that thelens does not include any antireflection treatment (the refraction indexof CaF₂ being low, the natural reflectivity of the material is low,whereas a bi-spectral antireflection treatment on this material isconsidered not to be very effective).

The triplet ZnSe(+)/Ge(−)/CaF₂(−) (solution 1) has been used.

But it is also possible to use the triplet ZnS(+)/Ge(−)/CaF₂(−)(solution 2).

The solutions 1/ and 2/ are equivalent. The solution 1/ is preferred tothe solution 2/ because the CaF₂ lens of solution 1 is less powerfulthan the CaF₂ lens of solution 2: therefore potentially, for a similarcost, the MTF of solution 1 is greater than the MTF of solution 2.

The chromatism was corrected on the group G1 mainly responsible for thechromatism. This correction may be enhanced by also correcting thechromatism of the other groups G2 and G3.

Since the value of the aperture radius is relatively small on each ofthe lenses L2, L3A and L3B of the groups G2 and G3 relative to thesemi-diameter of the entrance pupil, the contribution of these lenses tothe chromatism of the combination is intrinsically small. It istherefore sufficient to produce these components in a material that isnot very chromatic in each of the useful bands. The best candidate isgermanium.

L2, L3A and L3B are made of germanium. L2 and L3A are aspherical, L3B isspherical.

However, to improve the MTF, it is useful to reduce the chromatismintroduced by L3A. A perfect correction with a triplet is not necessary:it is possible to use a doublet of material. These doublets are computedbased on the following equations:

As above, φ is the power of the optical module, in this instance L3A,

$\quad\left\{ \begin{matrix}{{\phi\; A},{\phi\; B\mspace{14mu}{the}\mspace{14mu}{respective}\mspace{14mu}{powers}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu} 2\mspace{14mu}{optical}\mspace{14mu}{components}},} \\{{v\; 1A\mspace{14mu}{and}\mspace{14mu} v\; 2A\mspace{14mu}{the}\mspace{14mu}{IR}\; 2\mspace{14mu}{and}\mspace{14mu}{IR}\; 3\mspace{14mu}{constringencies}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{component}\mspace{14mu} A},} \\{{v\; 1B\mspace{14mu}{and}\mspace{14mu} v\; 2B\mspace{14mu}{the}\mspace{14mu}{IR}\; 2\mspace{14mu}{and}\mspace{14mu}{IR}\; 3\mspace{14mu}{constringencies}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{component}\mspace{14mu} B},} \\{{\lambda\; 1\mspace{14mu}{is}\mspace{14mu}{the}\mspace{14mu}{peak}\mspace{14mu}{sensitivity}\mspace{14mu}{wavelength}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{IR}\; 2\mspace{14mu}{band}},} \\{\lambda\; 2\mspace{14mu}{is}\mspace{14mu}{the}\mspace{14mu}{peak}\mspace{14mu}{sensitivity}\mspace{14mu}{wavelength}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{IR}\; 3\mspace{14mu}{{band}.}}\end{matrix} \right.$

ν1 and ν2 are the equivalent constringencies of the doublet in the IR2and IR3 spectral bands.

They are defined by the following equation:φ/νi=φA/νiA+φB/νiB where i=1 or 2.

The equations that make it possible to compute the powers φA and φB areas follows:

$\quad\left\{ \begin{matrix}{\phi = {{\phi\; A} + {\phi\; B}}} \\{{{1/\lambda}\; 1*\left( {{\phi\;{A/v}\; 1A} + {\phi\;{B/v}\; 1B}} \right)} = {{{- 1}/\lambda}\; 2*{\left( {{\phi\;{A/v}\; 2A} + {\phi\;{B/v}\; 2B}} \right).}}}\end{matrix} \right.$

These equations ensure that the normal axial chromatism difference willbe the same number of wavelengths in each of the two respective spectralbands.

The best candidate doublets for L3A are summarized in the table below.They are classified in ν1 order in decreasing absolute value.

{square root over (φA² + φB²)} A ν1A ν2A B ν1B ν2B φA φB ν1 ν2 0.99 IG4388 676 DOE* −4 −7 0.99 0.009 −25985 13967 0.99 GASI 395 472 DOE* −4 −70.99 0.01 −3198 1719 R1 0.99 GE 240 1900 DOE* −4 −7 0.99 0.01 1418 −7622.22 ZNS 186 97 ZNSE 322 232 −0.99 1.99 1157 −622 1.08 ZNSE 322 232 CAF233 13 1.08 −0.08 1070 −575 1.19 ZNS 186 97 CAF2 33 13 1.18 −0.18 982−528 1.05 CAF2 33 13 GASIR1 395 472 −0.05 1.05 937 −504 *DOE =Diffractive Optical Element

For correcting the chromatism of L3A, the best doublet is IG4/DOE. TheGe/DOE doublet is also a good candidate, by virtue of the highrefraction index of germanium.

For the ZnSe/ZnS doublet the powers of the lenses are considerable. Itmay be suitable for a group that is not very open, if it is desired toprevent diffractive elements.

The imaging system according to the invention may be incorporated intoan IR camera, into IR binoculars or into another item of IR optronicequipment. It may be installed in a tank gun sight, in a Forward-LookingInfra-Red system or FLIR, or in a pod installed on an aircraft.

FIG. 3 represents such an installation in a tank gun sight. The variousoperating modes of the camera (IR2 & IR3, NF & WF) have been superposedon the same figure. The tank gun sight 10 comprises a germanium window 4usually placed so as to prevent any narcissus effect, and agyrostabilized mirror 5. FIG. 3 makes it possible to understand why itis of value to make the imaging system upstream of the first system lenscompact: it makes it possible to reduce the size of the window 4 and ofthe gyrostabilized mirror 5.

It will be readily seen by one of ordinary skill in the art that thepresent invention fulfils all of the objects set forth above. Afterreading the foregoing specification, one of ordinary skill in the artwill be able to affect various changes, substitutions of equivalents andvarious aspects of the invention as broadly disclosed herein. It istherefore intended that the protection granted hereon be limited only bydefinition contained in the appended claims and equivalents thereof.

1. A dual-field (NF and WF) imaging system comprising an optronicdetector and an optical combination of narrow-field focal length F_(NF)having, on an optical axis: a front lens, a narrow-field entrance pupilsituated in the vicinity of the front lens, a real wide-field entrancepupil, situated upstream of the front lens, an intermediate focal plane(IFP), the optical combination comprising, on the optical axis, thefollowing refractive groups: a convergent front group G1 of focal lengthF, where F<F_(NF)/2, this group G1 comprising the front lens, adivergent field-change group G2 that can move along the optical axis,this group being situated upstream of the IFP in NF configuration anddownstream of the IFP in WF configuration, a relay group G3 imaging theIFP on the focal plane of the detector, wherein the detector is a cooledIR2/IR3 detector, and in that, since the refractive groups compriselenses, at least three different materials including CaF₂ are used forthe lenses of the front group G1.
 2. The imaging system as clamed inclaim 1, wherein the triplets ZnSe(+)/Ge(−)/CaF₂(−) orZnS(+)/Ge(−)/CaF₂(−) are used in the front group G1.
 3. The imagingsystem as claimed in claim 1, wherein the front group comprises 5lenses.
 4. The imaging system as claimed in claim 1, wherein the frontgroup comprises two lenses made of Ge, two lenses made of ZnS or ZnSe,and one lens made of CaF₂.
 5. The imaging system as claimed in claim 1,wherein the group G2 comprises one lens made of Ge.
 6. The imagingsystem as claimed in claim 1, wherein the relay G3 comprises lenses madeof Ge.
 7. The imaging system as claimed in claim 1, wherein the relay G3comprises at least one diffractive lens.
 8. The imaging system asclaimed in claim 1, wherein the relay G3 comprises at least one lensthat can move along the optical axis.
 9. The imaging system as claimedin claim 1, wherein the relay G3 comprises at least one microscanninglens making it possible to translate the image on the detector.
 10. Theimaging system as claimed in claim 1, wherein the detector is a linearor matrix detector.
 11. The imaging system as claimed claim 10, whereinthe detector is a multiple quantum well matrix detector.
 12. The imagingsystem as claimed in claim 1, wherein the imaging system has a coldscreen, the cold screen forms an aperture diaphragm.
 13. The imagingsystem as claimed in claim 1, wherein the imaging system has an aperturediaphragm, only this diaphragm limits the optical beams in order toprevent vignetting.
 14. The imaging system as claimed in claim 1,wherein the optronic detector is configured to capture an image of anobject upstream of the front lens.
 15. Binoculars comprising adual-field (NF and WF) imaging system comprising an optronic detectorand an optical combination of narrow-field focal length F_(NF) having,on an optical axis: a front lens, a narrow-field entrance pupil situatedin the vicinity of the front lens, a real wide-field entrance pupil,situated upstream of the front lens, an intermediate focal plane (IFP),the optical combination comprising, on the optical axis, the followingrefractive groups: a convergent front group G1 of focal length F, whereF<F_(NF)/2, this group G1 comprising the front lens, a divergentfield-change group G2 that can move along the optical axis, this groupbeing situated upstream of the IFP in NF configuration and downstream ofthe IFP in WF configuration, a relay group G3 imaging the IFP on thefocal plane of the detector, wherein the detector is a cooled IR2/IR3detector, and in that, since the refractive groups comprise lenses, atleast three different materials including CaF₂ are used for the lensesof the front group G1.
 16. A tank gun sight comprising A dual-field (NFand WF) imaging system comprising an optronic detector and an opticalcombination of narrow-field focal length F_(NF) having, on an opticalaxis: a front lens, a narrow-field entrance pupil situated in thevicinity of the front lens, a real wide-field entrance pupil, situatedupstream of the front lens, an intermediate focal plane (IFP), theoptical combination comprising, on the optical axis, the followingrefractive groups: a convergent front group G1 of focal length F, whereF<F_(NF)/2, this group G1 comprising the front lens, a divergentfield-change group G2 that can move along the optical axis, this groupbeing situated upstream of the IFP in NF configuration and downstream ofthe IFP in WF configuration, a relay group G3 imaging the IFP on thefocal plane of the detector, wherein the detector is a cooled IR2/IR3detector, and in that, since the refractive groups comprise lenses, atleast three different materials including CaF₂ are used for the lensesof the front group G1.